Double layer capacitors employing conductive solid and liquid electrolytes operate on a different principle from that employed in double-plate electrolytic capacitors. In these capacitors, a double layer of electrical charge occurs at the interface of the conducting solid and the liquid electrolyte. Typically, the conducting solid is immersed in a liquid electrolyte, such as a dilute sulfuric acid. A layer several atoms thick just inside the surface of the conducting solid constitutes one electrode of the capacitor, while the second electrode is a "virtual" electrode within the electrolyte. At the interface, a space of 1 to 2 Angstroms outside of the solid electrode contains the virtual electrode. A polarized layer of solvent molecules is attracted to the electric field of the electron layer. Outside the polarized (dipole) layer is a second charged area which contains ion particles in a diffuse layer extending about 25 Angstroms into the solvent bulk. Commercial double layer capacitors include two conductors separated by the liquid electrolyte and an ion permeable separator and are bipolar devices. The charge held in the capacitor is discharged through a load, using the ions in the electrolyte to complete the circuit between the virtual electrodes of the bipolar device. Without this ion-charged transfer path, the energy at the virtual electrodes would be inaccessible.
High capacitance is obtained in these devices by using activated charcoal for the solid electrode. The activated charcoal is a highly porous substance that has a very large surface area. For example, one gram of activated charcoal may have a surface area of 1,000 to 2,000 square meters.
Double layer capacitors, unlike batteries, can be cycled indefinitely and have at least a 10 year life. Further, these devices can easily be soldered into place, usually do not leak or explode, and easily indicate their charge. Further, they do not exhibit the undesired memory effect that is common to batteries such as Ni-Cd batteries.
Additionally, double layer capacitors can be operated over a wider temperature range than batteries. For example, these capacitors can be operated over a range from -25.degree. C. to +70.degree. C. and can be stored over a range from -40.degree. C. to +85.degree. C.
One drawback of the double layer capacitors employing a conducting solid and a liquid electrolyte, however, is the working voltage of such capacitors. The sulfuric acid solution commonly used in the commercially available double layer capacitors will decompose into hydrogen and oxygen with an electrical charge in excess of approximately 1.2 volts. Consequently, such a single double layer capacitor element has a maximum working voltage of only 1.2 volts. Should this working voltage be exceeded, there will be a chemical disassociation which can damage the device. With such a low working voltage, the double layer capacitor has limitations in applications.
To increase the voltage, such capacitors must be connected in series. However, such a connection has two drawbacks. First, the total capacitance decreases when connecting capacitors in series, and, further, the resistance increases which reduces the current handling capabilities by causing power dissipation or losses in both the charging and discharging modes. In production double layer capacitors, as many as six capacitive cells having 6 Farads of capacitance each are connected in series. This reduces the combined 36 farads of capacitance of the 6 cells to one farad, while raising resistance by a factor of six to create an acceptable working voltage for the higher voltage application.